My biggest piece of advice for kids that age is to take it easy. Remember there is no way you can do EVERYTHING in 1 trip.
Also, since you are going in December, I would recommend Mickey's Very Merry Christmas Party if you can do it. My young kids love that! That's the best time of year to go.
It's a great time of year to be at WDW. We'll be there the week after you. The weather can vary at that time of year so you'll need a variety of clothing. Dress in layers. The mornings tend to be cool, the afternoons usually warm up nicely and then the evenings get cool again. We actually had one night that we were downright freezing . Have a wonderful trip .